AC motorized wheel control system

ABSTRACT

Torque is distributed by calculating first and second torque commands using a requested torque and a ratio of speeds of first and second wheels and limiting them in accordance with respective torque command approved ranges and approved change rates; converting the limited torque commands to horsepower commands and limiting them in accordance with respective horsepower command approved ranges and approved change rates; and converting the limited horsepower commands to present torque commands. Maximum horsepower available is determined by using an engine speed to determine a nominal amount of available horsepower; applying a desired load status signal and an actual engine load status signal to a proportional-integral regulator; and using the nominal amount of available horsepower and an output signal of the regulator to determine the maximum amount of available horsepower. Thermal protection is provided by obtaining component temperatures of a plurality of components; normalizing each component temperature; obtaining a normalized drive system temperature by determining a maximum value of the normalized component temperatures; and comparing the normalized drive system temperature with at least one predetermined maximum normalized temperature and using a result of the comparison to determine whether a corrective action is needed. A truck is started on an incline by determining whether its speed is below a predetermined speed limit, a service brake is applied, and an accelerator pedal is depressed, and, if so, permitting a propulsion torque to build without requiring an operator override action.

This application is a division of application Ser. No. 08/923,484, filedSep. 4, 1997 now U.S. Pat. No. 5,939,846.

BACKGROUND OF THE INVENTION

The present invention relates generally to controls for drive systemsincluding AC motorized wheels. The present invention relates moreparticularly to torque distribution, engine loading, temperaturewarnings, and hill starting controls for haul trucks having drivesystems including AC motorized wheels.

Electric drive systems for haul trucks typically provide the same torqueto both rear wheels under all moving conditions. In some DC electricdrive systems, a foot pedal is mapped to engine speed which is relatedto horsepower in a nonlinear fashion. Conventionally, an engine loadcurve is a curve relating available engine horsepower to engine speedand is generalized for a nominal engine in nominal conditions. Thesedrive systems regulate engine speed to the full load speed by adjustingthe horsepower load only at steady state full throttle. For partialthrottle applications, the horsepower load is less than the optimalvalue to reserve engine power in case it is needed to accelerate theengine to full speed.

Conventional temperature warning indicators on electric drive haultrucks include warning lights for motor temperatures which exceedpredetermined limits. These temperature indicators generally mustcompromise between including either (1) a large number of temperaturereadings which an operator may not have time or ability to interpret or(2) a small number of readings which may not include some importantareas of the drive system such as the alternator or the exciter.

Electric drive haul trucks generally have retarding effort provided bythe electric motors and service brakes which use friction to stop thehaul truck. A single foot pedal is used to request retarder effort andservice brakes with the first half of the pedal travel controllingretarder effort and the second half of the pedal travel controllingfriction brakes. The friction brakes may also be activated by use of adash mounted switch which requests full braking effort. The drive systemrecognizes pressure in the friction brake lines as a request for fullretarder effort. When the drive system recognizes a retarder request, itprevents propulsion effort even if the vehicle speed is below the speedat which the retarder can operate (in one embodiment, for example, theretarder can only operate at speeds above one mile per hour). Haultrucks frequently have to start from a stopped position on steep uphillgrades (grades greater than or equal to about eight percent) with a fullload. If an operator releases the friction brakes before the drivesystem has developed enough torque to balance the weight of the truck onthe grade, the truck will roll backwards. During hill starts, frictionbrakes are used while the system is building torque to prevent the truckfrom rolling backwards and operators are required to depress an overridebutton to allow the system to propel against the friction brakes.

SUMMARY OF THE INVENTION

Thus there is a particular need for an AC electric drive system whichprovides more torque in partial throttle conditions during a turn to anoutside rear wheel than the inside rear wheel in order to minimize tirewear and turning radius; which provides control over the rate of torquechange to provide a smooth and controllable truck; and which providescontrol of the horsepower load on the engine to prevent the enginebogging down due to an overload. In one embodiment, the torque isdistributed in proportion to the left and right wheel speeds after beinglimited in the event that right or left wheel motor torque, right orleft wheel motor rate of torque change, total horsepower, or rate oftotal horsepower change would exceed its respective limit. Torque andtorque rate limits are useful for providing a smooth, controllablevehicle with minimized peak values (since high torque contributes togear wear), and horsepower rate limits are useful because the engine andalternator are limited in how fast they can change horsepower levels.

It would further be advantageous to have a drive system providing directdriver control of propulsion horsepower; optimal steady state loading ofthe engine during climatic, fuel quality, and engine conditionvariations over the complete engine speed range; and optimal loadcontrol during engine acceleration transient operation (to allow thedrive system to reduce power transiently to decrease the time to fullspeed and full horsepower). In one embodiment, an input from anaccelerator pedal is mapped through a shaping function directly topropulsion torque and horsepower instead of to an engine speed commandand thus provides good driver control of the propulsion effort. Theactual commanded horsepower is modified (based on a load signal from theengine and a nominal amount of horsepower calculated using a sensedengine speed) to a value which allows the drive system to optimally loadthe engine over the complete speed range. Additionally, this methodpermits the drive system to reduce the horsepower load while the engineis accelerating to reduce the time required to reach full horsepower.

There is also a need for an electric drive haul truck control systemincluding a comprehensive thermal self protection system with simplifiedoperator warning lights. In one embodiment of the present invention, thethermal self protection system represents the thermal state of the drivesystem as a single temperature instead of as a plurality of separatetemperatures. The system minimizes the required training of operatorsand provides operators with a concise display of the thermal state ofthe drive system.

It would be advantageous to further simplify haul truck driver tasks andtraining by permitting an operator to smoothly start on a hill withoutrequiring the operator to hold down an override or reset button. Thepresent invention interprets the natural sequence of driver actionsduring hill start situations to determine that a hill start exists andpermits the driver to perform a hill start without pressing additionalbuttons or performing difficult sequences of steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings, where like numerals represent like components, in which:

FIG. 1 is a block diagram of a curved road and a truck including frontwheels, left and right rear wheels, and left and right wheel motors.

FIG. 2 is a flow chart of a method of the present invention fordistributing torque between the left and right wheel motors.

FIG. 3 is a block diagram of a drive system control and its inputsignals.

FIG. 4 is a flow chart of a method of the present invention for optimalengine loading.

FIG. 5 is a block diagram of a thermal control system of the presentinvention.

FIGS. 6, 7, and 8 are side diagrams of foot pedals and a switch.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

TORQUE DISTRIBUTION

FIG. 1 is a block diagram of a curved road 138 and a truck 140 includinga right front wheel 142, a left front wheel 141, a right rear wheel 144,a left rear wheel 146, a right wheel motor 148, and a left wheel motor150, and FIG. 2 is a flow chart of a method of the present invention fordistributing torque between the left and right wheel motors. A firstmotorized wheel 546 includes wheel 146 and wheel motor 150, and a secondmotorized wheel 544 includes wheel 144 and wheel motor 148.

In step 110, the total available torque (tlimit₋₋ total₋₋ max) isobtained by obtaining the sum of the maximum available torque on eachwheel motor (tlimit₋₋ motor1₋₋ max+tlimit₋₋ motor2₋₋ max).

The torque available from each wheel motor is based on its respectivespeed. A computer can include software having a linear interpolationtable to look up the available maximum torque for each wheel motor basedon the speed of that individual wheel motor. The general slope of acurve useful in creating such tables uses two limits: (1) the availablemaximum torque must be below a predetermined level x, and (2) thehorsepower (which depends on the speed) must be below a predeterminedlevel y.

The relation between horsepower and speed is as follows:

horsepower=(tq*speed)/m

wherein tq represents torque and m represents a constant (which in oneembodiment wherein the torque is in lb.-ft and the speed is in rpmequals 5252). The curve is flat at level x from a speed of zero rpm to apredetermined speed (which in one embodiment is 286.6 rpm), and thecurve decreases by a factor of 1/speed above the predetermined speed toremain within a constant horsepower limit.

In step 112, a requested torque is scaled. In one embodiment, anoperator (driver) requests torque by applying pressure to a pedal 156(shown in FIG. 6) which has positions mapped between 0 and 1. Themaximum total available torque is then multiplied by the mappedrequested torque position to provide a scaled total requested torque.

In one embodiment, the mapped requested torque position from theaccelerator pedal is further used as a mapped requested horsepowerposition to request engine speed. In this embodiment, the mappedrequested horsepower position can be multiplied by the full loadhorsepower value (which in one embodiment is 2500 horsepower) to obtaincommanded horsepower. The commanded horsepower can be used with anengine load curve (a graph of the horsepower the drive system should usefrom the engine versus speed) to determine an appropriate engine speedcommand that could generate the commanded horsepower.

In step 114, a speed ratio between the left to right rear wheel speedsis calculated. When a vehicle is turning, the left and right front wheelspeeds are not the same, but a ratio of these speeds is not needed foruse in the present invention. The left and right rear wheel speeds(hereinafter referred to as the left and right wheel speeds) can beobtained by conventional speed sensors 344 and 346 such as tooth gearswith active quadrature tachometers which sense changes in reluctance.

In step 116, the speed ratio is used to calculate individual left andright torque commands (tqx₋₋ call1₋₋ now and tqx₋₋ call2₋₋ now) bydistributing the scaled total requested torque among the left and righttorque commands in proportion to the speed ratio between the left andright torque commands.

In step 118, it is determined whether either of the first and secondtorque commands fall outside their respective first and second torquecommand approved ranges, and, if so, in step 120, the appropriateindividual torque command is adjusted.

If either the left or right wheel motor torque limit (tlimit₋₋ motor1₋₋max, tlimit₋₋ motor2₋₋ max) is exceeded by a respective left or righttorque command, the torque for that motor will be set to its limit andthe torque to the other motor will be increased by the amount theopposite wheel motor exceeds its torque limit (up to its own limit).

The present invention provides more torque to the outside wheel at partthrottle than prior techniques where the torque was evenly distributed.The outside wheel will have a lower torque limit than the inside wheelbecause it is turning faster. As throttle increases from zero and thespeed ratio is held constant, torque is distributed based on the speedratio. As torque increases and reaches the outside wheel limit, theoutside wheel torque command is held constant and only the inside wheeltorque is increased, eventually exceeding the outside wheel torque.

Both motor torques should be limited to be above a minimum value basedon their respective motor speeds. Generally, motor torque will be at aflat torque level (flat₋₋ torquelevel) up to a predetermined motor speed(ramp₋₋ speed), and the motor torque will ramp down as the motor speed(motor₋₋ speed₋₋ rpm) increases above the ramp₋₋ speed by a factor of1motor₋₋ speed₋₋ rpm. For motor speeds less than ramp₋₋ speed, theminimum torque is flat₋₋ torquelevel. For motor speeds above the ramp₋₋speed and below a minimum torque level, the following equation can beused:

    (ramp.sub.-- speed/motor.sub.-- speed.sub.-- rpm)*flat.sub.-- torquelevel

In one embodiment, flat₋₋ torquelevel=750 lb.-ft, ramp₋₋ speed=500 rpm,and the minimum torque level=50 lb.-ft. The invention thus permits thesystem to provide the desired torque split at part throttle applicationsand still apply full propulsion effort at full throttle.

After the left and right torque commands are calculated (and, ifnecessary, adjusted), at step 122 it is determined whether thedifference between the requested torque for the right (or left) wheelmotor and the torque command for the right (or left) wheel of theprevious cycle is greater than a predetermined rate limit.

The rate limit between the present and previous torque commands is afunction of torque and the mode of the truck. In one embodiment, forexample, the increasing rate limit is 9000 lb.-ft/sec when the lasttorque command is less than 8000 lb.-ft, and the rate limit decreases to2500 lb.-ft/sec when the last torque command is greater than 8000lb.-ft. Typically, the truck does not require torques near the maximumtorque (about 19000 lb.-ft in one embodiment) to begin moving. A slowerrate of torque change allows the truck to start moving and then ride theengine horsepower limit before the torque reaches its maximum value.This method provides a smooth ride and minimizes wear on the gears.

The decreasing rate limit is set by the mode of the trucks and dependson whether the truck is exiting propulsion to enter retard versusreducing propulsion effort. In one embodiment, for example, when exitingpropulsion to enter retard the rate limit is set to--20,000lb.-ft/second, and when reducing normal propulsion effort the rate limitis set to--100,000 lb.-ft/second.

If a requested torque command for the left or right wheel motor exceedsthe limit, the assigned torque command is set to the previous cyclecommand plus the rate limit at step 124. If the other torque commanddoes not exceed the rate limit, its wheel motor torque command is notfurther adjusted.

At step 126, the resulting left and right wheel motor torque commandsare converted to horsepower commands, and at step 128, it is determinedwhether the horsepower commands are within predetermined limits.

Total maximum and minimum horsepower limits (hp₋₋ max and hp₋₋ min) canbe calculated as follows:

    hp.sub.-- max=minimum(hp.sub.-- old+rate.sub.-- limit.sub.-- max, hp.sub.-- available)

    hp.sub.-- min=hp.sub.-- old.sub.-- rate.sub.-- limit.sub.-- min

wherein: hp₋₋ available=horsepower available from engine 154, hp₋₋old=the sum of the last iteration torque commands converted tohorsepower values, rate₋₋ limit₋₋ max=the limit to the rate ofhorsepower change for increasing propulsion horsepower, and rate₋₋limit₋₋ min=the limit to the rate of horsepower change for decreasingpropulsion horsepower. In one embodiment, for example, rate₋₋ limit₋₋max=500 horsepower/second, and rate₋₋ limit₋₋ min changes according tothe mode of the system as follows: 100,000 horsepower/second if exitingpropulsion to enter a retard, 20,000 horsepower/second if exitingpropulsion because of a fault condition, and 4,500 horsepower/second forother reasons for exiting propulsion or decreasing horsepower.

Next, a horsepower ratio for the motors can be calculated with thefollowing equations:

    hp.sub.-- ratio=1/(1+temp)

    temp=(m2speed/m1speed)*((1-tq.sub.-- ratio)/tq.sub.-- ratio)

    tq.sub.-- ratio=tq1.sub.-- call.sub.-- old/(tq1.sub.-- call.sub.-- old+tq2.sub.-- call.sub.-- old)

wherein m1speed=the speed of motor 1, m2speed=the speed of motor 2,tq1₋₋ call₋₋ old=the torque call for motor 1 from the last iteration,tq2₋₋ call₋₋ old=the the torque call for the motor 2 from the lastiteration, and hp₋₋ ratio (which ranges from 0 to 1) represents thepercentage of horsepower which should be applied to motor 1.

Then horsepower maximums and minimums for the two wheel motors can becalculated as follows:

    hplimit.sub.-- motor1.sub.-- up=hp.sub.-- max*hp.sub.-- ratio

    hplimit.sub.-- motor1.sub.-- dn=hp.sub.-- min*hp.sub.-- ratio

    hplimit.sub.-- motor2.sub.-- up=hp.sub.-- max*(1-hp.sub.-- ratio)

    hplimit.sub.-- motor2.sub.-- dn=hp.sub.-- min*(1-hp.sub.-- ratio).

The hp₋₋ call for motor 1 is limited between hplimit₋₋ motor1₋₋ up andhplimit₋₋ motor1₋₋ dn, and the hp₋₋ call for motor 2 is limited betweenhplimit₋₋ motor2₋₋ up and hplimit₋₋ motor2₋₋ dn. If either horsepowercommand is not within its predetermined limits, at step 130, theindividual horsepower command is adjusted to be the upper limit if itexceeds the range or the lower limit if it falls below the range.

Thus the maximum total horsepower will be limited to hplimit₋₋ motor1₋₋up+hplimit-motor2₋₋ up, and the minimum total horsepower will behplimit₋₋ motor1₋₋ dn+hplimit₋₋ motor2₋₋ dn. Because of the manner inwhich the torque ratio is used in the above equations, the totalhorsepower rate of change will also remain within acceptable limits. Asshown by the above equations, the calculation to determine the rate ofchange limit includes determining the horsepower ratio (split) betweenthe two wheel motors which would match the torque split from the lastiteration and then determines what an acceptable change in horsepower isfor each wheel assuming the torque split is the same.

At step 136, the horsepower commands are converted to torque commandswhich are sent to an inverter controller.

The engine controller 155 is typically provided by the manufacturer ofengine 154 and has a location on the vehicle that is dependent on thetype of engine. Common positions for engine controllers are on theengine or in an auxiliary cabinet near the drive system controller.

FIG. 3 is a block diagram of one embodiment, for example, of a drivesystem 430 including a drive system controller 30 and its input signals.In FIG. 3, the drive system controller includes two separate computerswhich can exchange data. One computer 32 includes the propulsion systemcontrols (PSC) and is situated in a main cabinet. Signals supplied tothis computer include a signal from the engine controller and an enginespeed signal. The other drive system computer 34 is the truck controlinterface (TCI) computer which is located in an auxiliary cabinet. Thiscomputer receives signals from the acceleration pedal and suppliessignals to a temperature gauge. Each inverter controller may comprise aseparate computer 36 and 38 for receiving volt, amp, and speed data froma respective motor. If a more powerful computer were used, two or moreof these computers could be combined.

By calculating individual torque commands for the left and right wheels,the present invention provides more torque to the outside wheel whenpossible and prevents violations of the following limits: the totalhorsepower limit of the engine and the rest of the drive system, thetotal horsepower rate of change limit of the engine and the rest of thedrive system, individual motor torque limitations, and individual motortorque rate of change limitations and thus minimizes front tire wear,minimizes turning radiuses, and provides controllable propulsion andhorsepower load.

ENGINE LOADING

FIG. 4 is a flowchart for optimal engine loading for an AC wheel drivesystem. At step 211, an engine speed is sensed with an engine speedsensor 454 (shown in FIG. 1), and at step 220, an amount of nominalavailable horsepower can be determined using an engine load curve whichis a graph of the horsepower the drive system should use from the engineversus speed.

At step 214, an engine load status signal is obtained. In oneembodiment, the engine load status signal is a continuous analog signalfrom zero to ten volts created by an engine controller 156 whichindicates whether the engine 154 is underloaded or overloaded. A drivesystem controller 30 (shown in FIG. 3) reads the analog engine loadstatus signal after it is converted to a digital signal with an analogto digital converter (not shown).

At step 216, the load status signal is scaled and limited to anappropriate operating region. The determination of the operating regionis based on specific detailed characteristics about the signal which canbe obtained from the engine manufacturer.

There may be different limits depending on the particular engine and theway in which it generates load signals. For example, for engines fromsome engine manufacturers, it is useful to ignore load signals at lowengine speeds (such as below 900 rpm, for example), while engines fromother manufacturers may not have this limitation.

It is advantageous to permit the operator to be able command low levelsof torque by limiting the positive value of the engine load signal basedon pedal position. The limit can be computed with a 2 point table, forexample wherein (1) engine load is limited to be less than five voltswhen the pedal is depressed less than fifty percent of its range ofmotion, and (2) engine load limit is permitted to increase from 5 to 10as the pedal range increases from fifty percent to eighty percent. Inthis example, engine load is not limited when the pedal is pressedbeyond eighty percent.

At step 218, the scaled and limited load signal is used as an errorsignal input to a PI (proportional-integral) regulator with negativefeedback on the integrator. A PI regulator is a computer algorithmhaving an error signal input which is typically a set point signal(often the desired signal) minus a feedback signal (often the actualsignal) and an output signal which is used to adjust some aspect of thesystem to keep the error signal at zero. In the present invention, theengine load signal is used as the error signal.

In the AC Wheel application of the present invention, the output of thePI is a hp₋₋ adjust signal which adjusts the horsepower load of thedrive system on the engine. The PI regulator can adjust the drive systemhorsepower to keep the engine load signal at the desired value (which inone embodiment is about five volts).

The PI regulator output is the sum of a constant (Kp) times the errorplus the integral of a constant (Ki) times the error. The equation is:

    output=Kp*error+Integral(Ki*error)

In one embodiment wherein 5 volts is the desired value, the error is theload signal - 5. The constants Kp and Ki are set by experimentation or,if engine characteristics are available, can be calculated based onengine characteristics.

Negative feedback is not typically present in PI regulators. In thepresent invention, negative feedback (out₋₋ l) affects the integralportion of the equation as follows by modifying the PI equation:

    output=Kp*error+Integral(Ki*error-Knf*out.sub.-- i)

wherein Knf represents the negative feedback. Knf can be selectedmathematically by calculating how much steady state error would beacceptable to maintaining full reductions or additions of horsepower anddetermining a time within which the integrator should reset. Using thenegative feedback in this manner resets the state of the integrator whenit is not closing the loop.

Anti-windup limits the regulator action based on real limits of thesystem. A regulator only works properly when its output is actuallyaffecting the feedback it measures. Anti-windup is a standard part of PIregulators, and there are many different implementation schemes.Conventional anti-windup can be used with logic of the present inventionto turn off the integrator whenever the final torque command to theinverters does not reflect changes in hp₋₋ adjust.

A PI regulator is a type of closed loop controller in that the systemlooks at a feedback (an actual signal) to calculate a command in orderto achieve a goal of holding the feedback at a specific value. Hence,the controller is closing a loop. In the present invention, the integralterm can be turned off when the controller is not closing the loop dueto other system limitations controlling the amount of horsepower used.

At step 221, the output signal of the PI regulator from step 218 is usedas an offset to the nominal available horsepower found by step 220 todetermine the maximum horsepower available from the engine. For exampleif engine has 1810 nominal horsepower available at 1700 rpm and it is ahot day, the engine may be derating and actually only have 1750horsepower available. Thus if the drive system used 1810 horsepower itwould drag down engine speed, possibly until it stalls. The engine loadsignal provides information to the drive system the engine is overloaded(the voltage is <5 volts) This is a negative error because the set pointis 5 volts. The PI turns this negative error into a negative value whichbecomes more negative with time as the voltage stays less than fivevolts.

For example, assume there is -40 horsepower as the output of the PI (anhp₋₋ adjust signal). The drive system adds the hp₋₋ adjust (-40horsepower) to the nominal 1810 horsepower the load curve says isavailable, and thus the drive system believes that only a maximum of1770 horsepower is available. In the present example, after theadjustment of -40 horsepower, the engine would still be overloaded andkeep the load signal less than 5 volts until the PI output integratesdown to -60 horsepower at which point the drive system load and engineoutput match, and the engine load signal will then read 5 volts.

At step 222, the maximum amount of horsepower available is used as alimit on a commanded propulsion horsepower. If the engine loading methodof FIG. 4 is used in combination with the torque distribution methoddiscussed with respect to FIG. 2, the maximum amount of horsepoweravailable can be used by the algorithm in the torque control inventionto limit the horsepower command in step 128 of FIG. 2.

Thus, this method accomplishes optimal loading over the complete speedrange at steady state based on variations during acceleration byreducing load (power) to reduce the time required to reach fullhorsepower and full speed. There are three basic types of horsepowerversus speed curves: (1) the WOT (Wide Open Throttle) line which definesthe absolute maximum horsepower the engine can generate; (2) the optimalline defining the ideal horsepower the drive system should be using fromthe engine versus speed (at speeds less than full speed, it is usuallychosen for maximum efficiency, and at full speed it comes very close toor matches the WOT line); and (3) the drive system load curve which isthe amount of power the drive system should use versus engine speed forsteady state on a nominal engine.

As the driver increases the accelerator pedal from 0% to 100% the enginespeed increases and the drive system uses horsepower based on the drivesystem load curve. If the load signal and horsepower adjust of thepresent invention are not used, then the horsepower used to acceleratethe engine would be the WOT value minus the load curve value. In orderto get fast response from the engine (fast acceleration to full enginespeed) the load curve would be significantly lower than the WOT curve,and for most engines this would also make it lower than the optimalcurve. Thus if the operator were intentionally driving at 50%accelerator pedal--the engine would not be at optimal load.

With the load signal embodiment of the present invention, the load curvecan be set closer to the optimal curve line. If the engine is trying toaccelerate, it would indicate overload conditions and the drive systemwould decrease load and allow the engine to accelerate faster.

Furthermore, at steady state operation the load signal allows the drivesystem to adjust the horsepower used to match exactly what the enginecan do in the present conditions, not a nominal engine in nominalconditions as is represented by a nominal load horsepower curve.

THERMAL CONTROL

FIG. 5 is a block diagram of a thermal control system of the presentinvention including a computer 32 and a plurality (k) of components(shown as components 12, 14, and 16). The components represent differentcomponents of thermal interest in a drive system. In one embodiment, forexample, there are fourteen components (not shown) which include anauxiliary blower control phase controller, an auxiliary blower controlinverter, an alternator static exciter, an alternator field, left andright motor stators, left and right motor rotors, a dynamic brakingchopper GTO, a dynamic braking chopper diode, left and right inverterphase module GTOs, and left and right inverter phase module diodes.

A plurality (k) of temperature estimators (shown as 18, 20, and 22) canbe use to sense and/or use information to obtain component temperatures.These temperature estimators can either sense temperatures directly orcan calculate temperatures based on conventional mathematical devicemodels using various measured characteristics. For example, analternator field temperature model can use field current, engine rpm,ambient temperature, and barometric pressure to estimate temperature.Although estimators 18 and 22 are shown adjacent the components in FIG.5, some of the estimators such as estimator 20 which do not sensetemperature directly, can be included in computer 32. The estimatorswhich compute temperature instead of directly measuring it may usesensed parameters of the component itself as well as parameters obtainedfrom other sources.

For each component a respective warning temperature (Twarning) and arespective shutdown temperature (Tshutdown) is defined. The computerreceives temperatures from each of the estimators. The computer includesa normalizer 24 which interpolates each temperature between the ambienttemperature, its respective Twarning, and its respective Tshutdown toprovide an equivalent temperature on a normalized scale. The normalizedscale has a normalized warning temperature (NTwarning) and a normalizedshutdown temperature (NTshutdown).

The computer further includes a maximizer 26 which then determines themaximum value of all the normallized temperatures at any given time anddefines the drive system temperature to be that maximum value. The drivesystem temperature can be displayed on a monitor 28 for use by anoperator. although in the example of FIG. 5, an interface computer 34receives information from the propulsion system controlling computer 32for delivery to monitor 28, such an interface computer is not anecessary element of the present invention.

EXAMPLE OF NORMALIZATION CALCULATIONS:

In one embodiment, the following warning and shutdown temperatures areused

Twarning for GTO=105° C.

Tshutdown for GTO=115° C.

normalized ambient temperature (NTambient)=0° C.

NTwarning=60° C.

NTshutdown=85° C.

In this embodiment, as the drive system ambient temperature(Atemperature) changes, the normalization will change for GTOtemperatures (Ctemperature) less than 105° C. If the GTO temperature isless than the GTO Twarning, then the normalized GTO temperature(Ntemperature) can be calculated as follows: ##EQU1## Thus, if theambient temperature is 45° C., and if the GTO temperature is 75° C., thenormalized temperature would be ##EQU2##

Similarly, if the ambient temperature drops to 15° C., and the GTOtemperature is 75° C., the normalized temperature would increase to##EQU3##

Drive system ambient changes do not affect the normalization forcomponent temperatures at or above their respective warning temperature.In these situations, Ntemperature can be calculated as follows: ##EQU4##For example, if the ambient is 45° C. and the GTO temperature is 110°C., the normalized temperature would be ##EQU5##

If the GTO temperature is 120° C. (which is above the shutdowntemperature), the above formula would apply and the normalizedtemperature would be ##EQU6##

In one embodiment, the normalized temperature has an upper limit of 100°C. The values used as normallized warning, shutdown, and optional upperlimit temperatures are not significant and can be selected based onsafe, warning, and shutdown zone deflection percentages of the monitor.

As stated above, the present invention has an advantage of providing asingle temperature rather than a large number of separate temperatures.Thus, monitor 28 may comprise a single gauge which can show the singletemperature and/or a sequence of warning lights representative of thattemperature.

For example, if the highest normalized temperature is greater than orequal to the normallized shutdown temperature, a red light can be used.

Similarly, a yellow light can be used if the highest normallizedtemperature is between the normallized warning and shutdowntemperatures, and a green light (or, if preferred, no light) can be usedfor highest normallized temperatures falling below the normallizedwarning temperature.

If the operator sees a temperature number or a colored light, theoperator can take corrective action before the thermal limit whenoperation is restricted. Operator training can teach the operator aboutproper reactions to various temperature warnings. Additionally, ifdesired, the monitor can display instructions to the operator.

The computer can send instructions to a drive system controller 30(which may be a separate microprocessor or a portion of a microprocessorshared with the computer). For example, if a normalized shutdowntemperature is exceeded, the computer would inhibit the drive systempropulsion effort while permitting its retarding effort.

If a normallized warning temperature is exceeded during propulsion (butthe normallized shutdown temperature is not), an operator can observehow fast the temperature is rising and determine whether the truck canreach the top of the hill before the normallized shutdown temperature isreached. If the truck cannot, the driver can stop the truck at a safelocation (i.e. not in a switchback turn) and let the system cool down.During a retarding effort, the operator can reduce truck speed (by usingthe friction brakes) to generate less horsepower with the retardingsystem while maintaining speed and lower the temperatures. The drivesystem will continue to provide retarding effort as long as possible.

In one embodiment, the computer provides instructions to the drivesystem to automatically reduce the retard effort at a predeterminednumber of normalized degrees (5 for example) below the normallizedwarning temperature to a level at which the system is continuously rated(that is, a level at which the system should be able to run indefinitelywithout overheating at an ambient temperature of 55 ° C.). If theambient temperature is below this level, the drive system can providesignificantly more retard effort and thus allow the operators to retarddown the hills at higher speeds in cooler whether.

HILL STARTS

In the present invention, a computer is used to monitor three variables(1) truck speed (as obtained from a truck speed sensor 440 (shown inFIG. 1) which obtains an average of the speeds of the truck's fourwheels), (2) accelerator pedal application; and (3) friction brakeapplication. In some embodiments, the brake application will be by apedal 160 (shown in FIG. 7), and in other embodiments the brakeapplication will be by a switch 162 (shown in FIG. 8).

If friction brake application is controlled by a pedal and the truckspeed is less than a predetermined limit, then the computer looks for asequence of the driver (1) applying the service brake with his leftfoot, and (2) depressing the accelerator pedal 156 (shown in FIG. 6)with his right foot. In this situation, the computer interprets theseactions as a hill start and permits the propulsion torque to build. Theoperator can then slowly release the service brake while propulsiontorque builds.

If the brake application is controlled by a switch and the truck speedis less than a predetermined limit, then the computer looks for asequence of the driver (1) applying the service brake switch and (2)depressing the accelerator. Again, the computer interprets these actionsas a hill start and permits the propulsion torque to build. The operatorcan release the service brake when the operator believes drive systemhas enough torque to propel the vehicle up the hill.

The determination of the predetermined truck speed limit for thisanalysis preferably applies hysteresis. In one embodiment, for example,if the truck has been moving at a speed above one mile per hour, thetruck's speed must be reduced to below 0.3 miles per hour to be "lessthan a predetermined limit." If the truck has not been moving, then thetruck remains "less than a predetermined limit" until it reaches a speedof one mile per hour.

In the present invention, the drive system continues to ignore retarderrequests from the foot brake pedal and service brake pressure switch aslong as the truck speed remains below a second predetermined speed limit(above the first predetermined speed limit used to initially permitpropulsion torque to build) which, in one embodiment, is five miles perhour. This allows the operator to request propulsion effort bydepressing the accelerator pedal and thus drive through the frictionbrakes without additionally holding down an operator reset button. Whenthe service brake is controlled by the foot pedal, this method allowsthe driver to perform a smooth hill start by driving with one foot onthe brake and one foot on the gas and gradually releasing the brake asthe driver feels the drive system producing enough torque to keep thetruck from rolling backwards.

While only certain preferred features of the invention have beenillustrated and described herein, many modifications and changes willoccur to those skilled in the art. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

What is claimed is:
 1. A method for determining a maximum amount ofavailable horsepower in a drive system including two AC motorizedwheels, the method comprising:sensing an engine speed; using the speedto determine a nominal amount of available horsepower; obtaining anactual engine load status signal; applying a desired load status signaland the actual engine load status signal to a proportional-integralregulator; using the nominal amount of available horsepower and anoutput signal of the proportional-integral regulator to determine themaximum amount of available horsepower.
 2. The method of claim 1 furtherincluding scaling the actual engine load status signal, and whereinapplying the desired load status signal and the actual engine loadstatus signal to the proportional-integral regulator includes applyingthe desired load status signal and the scaled actual engine load statussignal to the proportional-integral regulator.
 3. The method of claim 2,wherein the scaled actual engine load status signal is limited to apredetermined operating range based upon accelerator position.
 4. Themethod of claim 3, wherein the scaled and limited engine load statussignal is used as an error signal input to the proportional-integralregulator.
 5. The method of claim 1 wherein the proportional-integralregulator includes an integrator portion with negative feedback.
 6. Themethod of claim 5 wherein the proportional-integral regulator includesanti-windup limits.
 7. The method of claim 1, wherein the actual engineload status signal is a continuous analog signal measured in volts thatis created by an engine controller such that, depending upon theobtained voltage, it can be determined whether the engine is underloadedor overloaded.
 8. The method of claim 1, wherein the engine load statussignal is used as a feedback signal and the desired load status symbolis used as a set point signal to the proportional-integral regulator. 9.The method of claim 8, wherein the proportional-integral regulatorcomputes an error signal equal to the set point signal minus thefeedback signal and wherein the output signal from theproportional-integral regulator can be used to adjust the drive systemto keep the error signal near zero.
 10. A load system for determining amaximum amount of available horsepower in a drive system including twoAC motorized wheels, the load system comprising:an engine speed sensorfor sensing an engine speed; and a computer forusing the speed todetermine a nominal amount of available horsepower; obtaining an actualengine load status signal; applying a desired load status signal and theactual engine load status signal to a proportional-integral regulator;and using the nominal amount of available horsepower and an outputsignal of the proportional-integral regulator to determine the maximumamount of available horsepower.
 11. The load system of claim 10 whereinthe computer is further adapted to scale the actual engine load statussignal and to apply the desired load status signal and the actual engineload status signal to the proportional-integral regulator by applyingthe desired load status signal and the scaled actual engine load statussignal to the proportional-integral regulator.